The invention relates to a vacuum cooling device for foodstuff in particular bakery products which is suitable for condensation of a vapor, in particular of water vapor. A vacuum cooling device for foodstuff, in particular bakery products, for example freshly baked bread under a negative pressure comprises a vacuum chamber, which is configured for receiving the fresh foodstuff for the purpose of cooling; a vacuum source, for example a vacuum pump which is connected with the vacuum chamber to discharge the vapor from the vacuum chamber and to generate a negative pressure in the vacuum chamber and a vapor condenser.
It is known to provide devices for the condensation of solvent vapors of vacuum distillation apparatus or vacuum evaporators from DE 1 017 593 B. Such a device contains a tube coil or a tube bundle in which the vapors to be condensed circulate. The tube coil is directed into a receiving vessel, which is used for receiving the condensate. If the vapors of the tube bundle are not condensed completely by the water bath, the vapors collecting in the receiving vessel are condensed in an auxiliary condenser. This condenser is also arranged in the water container inside a displacer. The auxiliary condenser can contain an absorption means, by which humidity is absorbed. Due to the fact that it relates to solvent vapors, which should be condensed by this device, it is mandatory that the cooling medium and the solvent vapors to be condensed don't come into contact with each other. Such a requirement doesn't apply to foodstuff, therefore this device is not suitable for the vacuum cooling of foodstuff.
A condenser for high pressure vapor is known for vapor engines from WO97/32113, which consists of a pressure vessel, in which a vapor is introduced by a vapor supply conduit. The vapor is introduced by a distributor directly into the water bath. The distributor has a plurality of distribution tubes which extend into the water bath. The distribution tubes are in contact with the water bath via a plurality of small openings, such that vapor is introduced through the openings into the water bath in the form of bubbles, where its condensation occurs.
Hot and humid foodstuff can be cooled very efficiently in a vacuum chamber that means only a short period for cooling is required. However during the evacuation process a very large volume of vapor is generated, which can amount to a multiple of the chamber volume. In a vacuum chamber, which has a chamber volume of 4 up to and including 6.6 m3 the vapor volume can amount up to be a hundred fold of the chamber volume.
The capacity of the vacuum pump is determined by the vapor volume. That means the vacuum pump has to be configured such that the entire vapor volume leaves the vacuum chamber via the vacuum pump. An example for a vacuum pump 1 for a vacuum cooling chamber 2 is shown in the document JP11-211314. The vacuum pump 1 is designed according to the principle of a venturi pump, which sucks the air from the vacuum cooling chamber by the water pressure of the water flowing through the venture nozzle of the venturi pump. According to the principle shown in JP11-211314 the air from the vacuum cooling chamber is received in a water flow and transported away. No vapor condensation is performed by this method. The power of the vacuum recirculation pump 8, which is needed for operating the venturi pump 1 has to be dimensioned for the entire air flow containing the vapor and has thus to be of a large size. In order to reduce the amount of vapor which has to be handled by the vacuum pump, vapor condensers are added to the vacuum conduits. An example for such a solution is shown in the U.S. Pat. No. 2,696,775. U.S. Pat. No. 2,696,775 shows a vacuum chamber for the cooling of foodstuff, which is combined with a baking oven. After conclusion of the baking process the bakery product is evacuated. Therefore the vapor is evacuated by a conduit, enters a condenser and then a vacuum pump. The condenser is arranged between the vacuum chamber and the vacuum pump and does not form the bottom of the vacuum chamber, such that the vapor is not in contact with the water bath. A further example for a heat exchanger in which a condensation occurs arranged upstream of a venturi pump is shown in JP2001-221546. Such vapor condensers prevent that a portion of the vapor enters the vacuum pump connected downstream thereto, however the vacuum pump has to cope with the entire non condensed vapor volume flow from the vacuum chamber.
Document DE 2902270 refers to a method for cooling a bakery product by decrease of pressure in a cooling chamber. The vapor is sucked from the cooling chamber by means of a vacuum pump. The control of the cooling is such that the pressure is reduced to a set value and in a second phase of pressure reduction to the next lower set value if the measured pressure during a given time interval Δt doesn't exceed the set value of more than a given value Δp. Bakery products of oven temperature can be cooled within a couple of minutes to environment temperature, such that it is resistant to cut and does not form a condensate anymore when packed in plastic bags.
The solution proposed in DE 2902270 causes no change in the capacity requirement of the vacuum pump, it merely allows a more precise control of the vacuum cooling process. A more precise control of the vacuum cooling process can lead to a decrease of the operating period. Thereby the total energy requirement for the vacuum cooling device per batch can be decreased, however, this has none or only a minimal effect on the power required for the vacuum pump.
The document DE 2 301 807 relates to a method for cooling and/or drying of humid foodstuff by means of a vacuum, whereby each of the following are considered for control: the temperature of the foodstuff or the atmosphere surrounding the foodstuff, the pressure the degree of atmospheric humidity or the humidity of the foodstuff after cooling, the degree of saturation of the atmosphere or the time. Thus also this document relates to an improvement of the control of the vacuum cooling device, which takes a multitude of parameters into account.
In the document U.S. Pat. No. 2,072,737 there is described a method for cooling of bakery products in which the crust of the bread is to be held soft and wet. The hot baked bread is cooled during a first period, such that the crust is cooled in portion. Then the bread is cooled further, in that it is introduced into a vacuum surroundings, whereby the vacuum is set such that it is maintained above the vapor pressure of the bread at bread or bread crust temperature.
The document DE 10 2008 035 068 A1 relates to a process for the manufacture of bakery products or fried products, in which a high relative humidity is maintained during the entire dough processing and dough handling and the finished baked hot bakery product is submitted to a vacuum cooling process. The relative humidity is greater or equal to 96%.
AT4002798 describes a method for manufacturing and conservation of prefabricated bakery products. The dough elements are baked partially and are cooled under shock to below 0° C. The dough elements contain 5-15% more water compared to the amount of flour as well as additives binding water. For this reason the baking time can be reduced to about 15-45% and the baking temperature increased to about 5 up to and including 25%. In the hot state, the baking products are subjected to a negative pressure.
Document WO2012082060 describes a vacuum cooling plant for bakery products, which is humidified with low pressure vapor. The device comprises a vapor generator for generating vapor under a sub-atmospheric pressure. The vapor is substantially free from air, whereby the vacuum generator is arranged in connection with the vacuum chamber, such that the generated vapor enters the chamber without contemporaneous air supply.
Thus, in the prior art, nearly the entire water vapor is discharged into the environment by the vacuum pump. As a consequence the throughput through the vacuum pump is increased up to 100 times the volume of the through-put of air. A portion of the water vapor can be used specifically to increase the humidity of the bakery products. This solution is subject to a high energy consumption. Thus a vacuum pump with high capacity has to be used. The power of the pump is usually about 45 kW. The use of such a high capacity vacuum pump has a number of disadvantages in operation next to the high energy consumption.
In the most cases the vacuum pumps to pump the vapor volume are of the oil-lubricated type, due to the fact that they have a high electrical connection value. During the operation of the vacuum pump the vapor which has to be discharged from the vacuum chamber comes into contact with the lubricant. If the vapor condenses before entering the vacuum pump or condenses in the interior of the vacuum pump, a water-oil mixture forms, which forms a stable emulsion. The lubricant has to be heated substantially above the boiling point of water to avoid forming of such an emulsion. The lubricant has a boiling point of above 100° C. The boiling point of the water lies below. The vacuum pump should therefore be operated above the boiling point of water. Thus the entire vacuum pump parts which come into contact with the vapor should have a temperature with is above the boiling point of water that means the water vaporizes under atmospheric conditions. For this reason the vacuum pump has to keep running also when it is not needed for generating a vacuum to generate the necessary heat. An oil emulsion can appear if vapor is in the vacuum chamber, even if the vacuum chamber is not anymore used for performing a vacuum cooling. Thus the vacuum pump should run at least for a further hour after the last vacuum cooling process has been completed. The vacuum pump should be started at least an hour, advantageously 90 minutes before performing a vacuum cooling process to obtain the necessary operating temperature.
A separator can be arranged downstream of the vacuum pump to condense the water vapor. If water vapor condenses already in the vacuum pump, this water vapor is enriched with lubricant. For this reason sticky lubricant residues remain in the water vapor condensate, which can reach the water discharge. Due to the fact that the oil emulsion which is formed by the condensing water vapor in the vacuum pump is stable, it is under certain circumstances not possible to separate it by a conventional oil filter. The separators to be used are bulky, difficult to dewater and to clean.
The use of a powerful vacuum pump can also lead to a phenomenon, in which contaminants, such as dirt particles, fats resulting from the foodstuff or other baking residues are evacuated from the vacuum chamber. Filters can be built in the vacuum conduit between the vacuum chamber and the vacuum pump to prevent that such contaminants from reaching the vacuum pump. However, each filter in the vacuum conduit increases the pressure drop, which is to be considered as of negligible influence in a pressure region of 200 to 300 mbar however, for pressure below 20 mbar, in particular below 5 mbar and below a substantial increase of the pump capacity is required to obtain an evacuation in these lower vacuum ranges.
The condensate of the vapor evacuated from the vacuum chamber is acidic and any machine parts of the vacuum pump made of steel can corrode, if they come into contact with the condensate. Vacuum pump parts as well as filters or separators made of steel can corrode, if they come into contact with the condensate. It is possible to use vacuum pump parts as well as filters or separators made of stainless steel. However it was shown, that also parts made of stainless steel can corrode if condensate accumulates in a dead space. In order to keep the oil temperature all the time above the boiling point of the water, the vacuum pump has to be switched on all the time. Thus an additional energy consumption is required because the vacuum pump has to be switched on all the time, even if it is not needed for the vacuum cooling process.
The progress of the cooling process can influence the quality of the vacuum cooled foodstuff. The progress of the cooling process is defined and monitored in particular by the control of the pressure in the vacuum chamber and optionally a control of the temperature. According to the prior art the pressure control is performed through a control rule which maps progress of pressure over time, that means a pressure curve. The pressure curve is determined by the recipe for the preparation of the foodstuff. Until present the pressure had to be controlled by a valve by a two point control. The valve has substantially two operating states, the closed state in which the connection to the vacuum pump is interrupted and the open state in which a connection to the vacuum pump exists and vapor can be evacuated from the vacuum chamber. That means one of the points of the two point control corresponds to the closed valve, a second one of the points of the two point control corresponds to the opened valve. If the valve opens the pressure is reduced abruptly. This abrupt reduction of pressure may lead to escaping of moisture from the foodstuff which enters the vapor cycle. For this reason various types of foodstuff can't be cooled optimally by this two point control. As a consequence a decrease in quality of the foodstuff has to be accepted. The vacuum pump needs to operate for this simple two point control continuously without interruption.
An object of the invention is to develop a vapor condenser and a vacuum cooling device with a vapor condenser by which a large portion of the vapor generated in the vacuum chamber can be condensed, without risking a contamination of the vacuum pump. It is a further object of the invention to decrease the power requirement of the vacuum pump.